1. Field of the Invention
This invention relates generally to dual-tone multi-frequency (DTMF) telephone dialing systems, and relates more particularly to a method for approximating sinusoidal waveforms with trapezoidal waveforms, and to a general purpose digital processor adapted for generating DTMF dialing signals.
2. Description of the Prior Art
Touch-Tone and other tone dialing telephones utilize dual-tone multi-frequency (DTMF) dialing signals. Dual-tone multi-frequency (DTMF) dialing signals are composed of two simultaneous signals having sinusoidal waveforms of different frequencies. Each key on a telephone keypad is designated by a combination of two sinusoidal waveforms. The lower frequency sinusoidal waveform corresponds to one of four rows on the telephone keypad, and the higher frequency sinusoidal waveform corresponds to one of three columns on the telephone keypad. A fourth column frequency is provided for future use, bringing the total number of frequencies to eight. Since all of the frequencies are within the audible range, the sinusoidal waveforms are also known as tones. Together, two tones designate one key of the keypad.
DTMF tone generators are utilized in telephone systems to generate DTMF dialing signals. A DTMF tone generator is coupled to the keypad to receive information regarding key closures, and is capable of generating combinations of the eight tones. When a telephone key is pressed, two tones are generated by the tone generator, one corresponding to the row of the key and the other corresponding to the column of the key.
In the past, most DTMF tone generators have been analog devices. More recently, digital circuits and digital/analog circuits have been utilized as DTMF tone generators. One typical digital/analog integrated circuit DTMF tone generator is American Microsystems' model S25089 tone generator. The S25089 circuit digitally synthesizes the dialing signals, and then converts them to analog signals. In operation, the circuit divides a high frequency clock signal by a programmable amount, depending upon the key depressed, to provide a timing signal to a counter. The timing signal has a frequency equal to a known multiple of the frequency of the tone to be generated. The counter counts up and down, as clocked by the timing signal, with one full counting cycle defining one period of the synthesized signal. The counter selects taps on a weighted resistor ladder network, which converts the digital output of the counter into an analog voltage signal that approximates a sinusoidal waveform. The S25089 circuit has two parallel tone generator circuits, one for each tone. The signals from each parallel circuit are combined and buffered to generate the DTMF dialing signal.
A major disadvantage of the DTMF tone generators like the S25089 is that they are dedicated circuits with limited flexibility. While such DTMF tone generators do generate dialing signals that meet telephone industry specifications for DTMF dialing signals, that is basically all they can do. Another disadvantage is that they are not readily adaptable to the generation of digital, rather than analog, dialing signals, since they rely on the weighted resistor ladder network to generate signals having approximately sinusoidal waveforms.
General purpose digital processors can be utilized to generate digital DTMF dialing signals. A digital DTMF signal is a time varying digital representation of two simultaneous sinusoidal waveforms. Two computational schemes have been used to generate digital DTMF dialing signals: memory table look-up, and Maclaurin series expansion.
In the memory table look-up method of generating digital DTMF dialing signals, the trigonometric values of one or more sinusoidal waveforms are stored in memory. At each sampling instant, a value is taken from the table and is scaled to produce the digital signal. If the available memory is sufficiently large, full tables for each of the dialing signal frequencies can be stored. Smaller memories may be used, at the cost, however, of increased computation to derive the signal. A major disadvantage to the memory table look-up method is that a memory device of substantial size must be dedicated to DTMF tone generation, as well as a computational device such as a programmable processor. Although some savings in memory size can be achieved, it comes at the cost of increased processor overhead time.
In the Maclaurin series expansion method of generating digital DTMF signals, extensive use of the computational power of a processor is required. At each sampling instant, a value is calculated according to the formula: EQU cos (x)=1-x.sup.2 /2!+x.sup.4 /4!-x.sup.6 /6!+. . .
Alternatively, the value can be calculated from an equivalent sine series. The number of terms used depends upon the time and computational power available and the accuracy required. While this method requires less memory than the table look-up method, it significantly burdens the processor.